This invention relates generally to printed wiring board and, more particularly, to a technique of forming printed wiring boards and the resulting board to provide built-in inspection aids based on the criticality of the various device joining structures on the printed wiring board.
Present day printed circuit boards have many types of discrete components attached or mounted to them. Some components are surface mounted to metal features, usually formed with a photolithographic process using various techniques, such as solder, wire bond, mechanical compression, as well as other techniques, while others are inserted into holes previously drilled and metal plated. In most cases, there are a multitude of connection points between the component and the printed circuit board on some fixed and repetitive grid patterns. A typical grid may be a matrix of pads, such as a ball grid array, as well as others, with centerlines on the order of 0.050 inch to 0.100 inch apart. Much testing and inspection of these features at many different stages is performed to insure that they meet a predetermined defect criteria for items, such as metal voiding, missing metallization, size, contamination as well as may other criteria. The past practice has treated all the features in the grid with the same degree of criticality during the board inspection process, and defective parts are either scrapped or repaired according to quality specifications which are of the highest quality. Connections on the I/C chips are typically provided for all the attachment structures in the grid.
However, all of the connections on the chips and, hence, all of the corresponding features or attachment structures in a specific grid array may not be actually electrically functional or may have a redundant function to others on the same chip or printed circuit board. In fact, in many grid arrays of component mounting features, there are more non-critical features than critical features. Generally speaking, a non-critical feature is one where the pad is not used for electrical function or is used for a redundant function for other features on the pad. It should be noted that while many of the connections may be less critical from an electrical or circuit point of view, nevertheless in the design of the circuit board, the features are included for several reasons. First, they may be necessary for providing structural integrity to the chip. Second, the features may be necessary for future use, such as engineering changes. Thus, they are present.
Which mounting features or mounting structures are critical and which are not critical is not readily obvious to the equipment or operator inspecting the board. Therefore, defects in non-critical features frequently result in the board being scrapped or repaired unnecessarily. In high volume manufacturing environments, processing many part numbers with different types of component mounting scenarios and varying levels of human expertise, mapping of each individual part number""s functional features is not practically accomplished. Therefore, there is a need to identify which features or mounting structures are critical and which are not during the inspection phase, so that the boards need not be rejected because of defects in non-critical features or mounting structures.
According to the present invention, a wiring board for mounting an electrical device, which has an array of connectors thereon arranged in a grid pattern, is provided, wherein the connectors have at least two levels of criticality of connection to the substrate. The substrate has a plurality of mounting structures or features arranged in the same grid pattern to connect with the array of connectors on the electrical device. The mounting structures or features are divided into a plurality of at least two groups, with each group corresponding to a level of criticality of the connectors on the device. Each group of mounting structures has a discernible feature differing from each other group, to thereby permit different levels of inspection criteria for each group. The invention also contemplates a method of forming a substrate having the features for connecting the connectors on the device that have different levels of criticality.